为什么在C ++ 11或C ++ 14中没有置换迭代器? [英] Why no emplacement iterators in C++11 or C++14?
问题描述
C ++ 98有 front_inserter
, back_inserter
和插入器
,但是在C ++ 11或者草稿C ++ 14中似乎没有任何这些的位置版本。是否有任何技术原因,我们不能有 front_emplacer
, back_emplacer
和 emplacer
?
C++98 has front_inserter
, back_inserter
, and inserter
, but there don't seem to be any emplacement versions of these in C++11 or draft C++14. Is there any technical reason we couldn't have front_emplacer
, back_emplacer
, and emplacer
?
推荐答案
有任何技术原因我们不能有front_emplacer, back_emplacer和emplacer?
Is there any technical reason we couldn't have front_emplacer, back_emplacer, and emplacer?
不,没有技术原因。作为证明,这里是一个完整的实现 back_emplacer
演示您的用例1 ...
No, there is no technical reason. As proof here is a complete implementation of back_emplacer
with a demo of your Use Case 1...
#include <iterator>
#include <vector>
#include <iostream>
template<class Container>
class back_emplace_iterator : public std::iterator< std::output_iterator_tag,
void, void, void, void >
{
protected:
Container* container;
public:
typedef Container container_type;
explicit back_emplace_iterator(Container& x) : container(&x) {}
template<class T>
back_emplace_iterator<Container>&
operator=(T&& t)
{
container->emplace_back(std::forward<T>(t));
return *this;
}
back_emplace_iterator& operator*() { return *this; }
back_emplace_iterator& operator++() { return *this; }
back_emplace_iterator& operator++(int) { return *this; }
};
template< class Container >
inline back_emplace_iterator<Container>
back_emplacer( Container& c )
{
return back_emplace_iterator<Container>(c);
}
struct Demo
{
int i;
Demo(int i) : i(i) {}
};
int main()
{
std::vector<int> x = {1,2,3,4,5};
std::vector<Demo> y;
std::copy(x.begin(), x.end(), back_emplacer(y));
for (auto d : y)
std::cout << d.i << std::endl;
}
可能的已知问题:运算符的通用引用=
隐藏隐式生成的复制/移动 operator =
?如果是这样,需要以一种在重载解析中通用引用的方式进行显式定义。
Possible Known Issue: Does the universal reference of operator=
hide an implicitly generated copy/move operator=
? If so these need to be explicitly defined in a way that beats the universal reference in overload resolution.
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